linux-kernel - Re: [PATCH v12 10/12] namei: aggressively check for nd->root escape on ".." resolution

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Message-ID: <20190904232911.GN1131@ZenIV.linux.org.uk>
Date:   Thu, 5 Sep 2019 00:29:11 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Howells <dhowells@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>, Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
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        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
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Subject: Re: [PATCH v12 10/12] namei: aggressively check for nd->root escape
 on ".." resolution

On Wed, Sep 04, 2019 at 03:38:20PM -0700, Linus Torvalds wrote:
> On Wed, Sep 4, 2019 at 3:31 PM David Howells <dhowells@...hat.com> wrote:
> >
> > It ought to be reasonably easy to make them per-sb at least, I think.  We
> > don't allow cross-super rename, right?
> 
> Right now the sequence count handling very much depends on it being a
> global entity on the reader side, at least.
> 
> And while the rename sequence count could (and probably should) be
> per-sb, the same is very much not true of the mount one.

Huh?  That will cost us having to have a per-superblock dentry
hash table; recall that lockless lockup can give false negatives
if something gets moved from chain to chain, and rename_lock is
first and foremost used to catch those and retry.  If we split
it on per-superblock basis, we can't have dentries from different
superblocks in the same chain anymore...